The present invention relates to engineered railroad ties, methods of manufacturing same, and methods of using same. In particular, the present invention relates to railroad ties which resist sliding in the ballast of a railroad bed.
Railroad ties serve to support rails and also to maintain proper distance between rails under expected loads. Failure to adequately serve either of these roles can lead to a derailment, endangering both lives and property.
In general, a railroad tie must be able to maintain the desired distance between and under a lateral load of 24,000 lbs., a static vertical load of 39,000 lbs., and a dynamic vertical load of 140,000 lbs. Thus, for a typical railway wherein the distance (gauge) between the rails is 56.5 inches, the ties must be able to maintain this distance without increasing by more than 0.125 inches, under the expected temperature and load variations, so as to prevent derailment.
To effectively withstand such loads, the tie material must possess both stiffness and strength. In this regard, a railroad tie should, in general, exhibit the following minimal physical properties:
compression modulus:at least about 172,000 psiflexural modulus:at least about 172,000 psicompression yield stress:at least about 3,000 psicompression strength:at least about 3,000 psiflexural strength:at least about 3,000 psi
Another factor regarding maintaining the proper distance between rails is thermal expansion. To be suitable as a railroad tie, the material must exhibit a low thermal expansion. Preferably, the material has a coefficient of thermal expansion of less than 6×10−5 in/in·° F.
Ties are exposed to large temperature variations, excessive amounts of ultraviolet light, severe weather conditions, attack from microorganisms and insects, and stress imposed by use. Thus, to prevent the occurrence of accidents, the materials used for manufacturing railroad ties need to be stiff, strong and resistant to ultraviolet light, temperature fluctuations, and microbe/insect attack.
Also, the material used for ties should be nonconductive to preclude electrical flow between the rails. For example, for freight railways, electrical signals are sent through the rails for purposes of communication between the front and back of the train. For passenger railways, electrical power is often sent through the rails themselves. Therefore, to prevent electrical shorts between the rails, the ties supporting the rails should be made from nonconductive materials.
The tie material should also be durable to avoid deterioration due to abrasion during use. For example, one form of abrasion associated with railroad ties is tie seat abrasion. This occurs when the tie plates cut into the ties. Ties that are made from materials that are stiffer and stronger than wood in the direction perpendicular to the tie axis are better at alleviating tie seat abrasion.
Since the rails are to be attached to the ties, the tie material also has to be suitable for use with typical types of fasteners, such as those used for wood materials, e.g., nails, screws, spikes, bolts, etc.
Typically, railroad ties are manufactured from wood, and to some extent steel-reinforced concrete. While wood is a relatively inexpensive material, it is very susceptible to attack from microorganisms such as fungi and insects, which will weaken and deteriorate the tie. To compensate for this, wooden railroad ties are often subjected to chemical treatments such as creosote treatment and chromate/copper/arsenic treatment. These treatments greatly increase costs. Further, chemical treatments only delay attack, not prevent it. Such treated woods also raise environmental concerns. Wooden ties are also quite susceptible to damage from harsh weather conditions and excessive sunlight. As a result of these drawbacks, wooden ties require frequent replacement or regauging, again increasing costs, in materials, labor, and disposal. Replacement and/or regauging costs can be quite substantial as ties are being utilized in numbers of about 3000 ties per mile.
Similarly, steel-reinforced concrete railroad ties are also susceptible to degrading forces, for example, abrasion, stress and strain. In fact, concrete ties have been found to cause premature failure of rails. This is because concrete ties are generally very stiff. As a result, when placed at the standard distance, the ties do not aid in absorbing the stress imposed on the rails thereby forcing the rails to flex more between the ties under load. To address this problem, concrete ties are often spaced closer together than wooden ties. This, of course, leads to increased costs.
Damp and freezing weather conditions cause damage to both wooden and concrete railroad ties alike. Water from rain or snow can penetrate into the surface of a wooden or concrete railroad tie. If the tie is then exposed to freezing conditions, the water will expand as it freezes, causing the formation of cracks thereby weakening the tie. In the case of reinforced concrete ties, such cracks can also lead to oxidation of the reinforcement bars.
Several attempts have been made to manufacture railroad ties from other materials, particularly polymeric and polymeric composite materials, which ameliorate the disadvantages associated with wooden and concrete ties. For example, Murray, U.S. Pat. Nos. 5,094,905 and 5,238,734, discloses making railroad ties from recycled tires. Neefe, U.S. Pat. Nos. 4,997,609 and 5,055,350, uses compression molding to manufacture a composite railroad tie from sand and granulated recycled plastics. These two components are held together by an adhesive coating material, i.e., sugar or polystyrene.
Nosker et al. (U.S. Pat. No. 5,789,477) describes railroad ties made from a composite containing coated fibers, such as fiber glass or carbon fibers, distributed within a polymer component containing about 80–100% high density polyethylene (HDPE). The polymer component can be made from recycled plastics.
Morrow et al. U.S. Pat. No. 5,298,214, hereby incorporated by reference, describes a material in which polystyrene is blended with a “mixed plastics” component from a recycling stream to produce materials that behave mechanically and appear morphologically like fiber reinforced composites. In this morphology, both the polystyrene component and predominantly polyolefin component, obtained from the “mixed plastics,” exist as a dual phase microstructure. Both components form three dimensional networks that are integrated and which interpenetrate with one another. Use of this material for railroad ties is described in the recent patent Nosker et al. U.S. Pat. No. 6,191,228, also hereby incorporated by reference.
A disadvantage exhibited by new wooden ties, as well as ties made from alternative materials, such as reinforced concrete ties and the plastic and plastic composite ties described above, is slippage within the ballast that forms the bed on which the ties lie. Due to their smooth surfaces, these ties tend to slip within the ballast as a result of the forces imposed on the ties.
This effect is most pronounced in curves. In such cases, there are two primary factors which can cause the ties to slip or push out of the ballast. First, as a result of high temperatures, thermal expansion of the rails and/or the ties can cause slippage, on sharp curves, especially in the case of the newer welded rails. Second, the centripetal acceleration of a train when traversing a curve imposes additional force on rails. These two factors tend to force or push the ties out of the ballast towards the outside of the curve. Most of the serious single train accidents in the past century can be blamed on these effects.
New wooden ties and ties made from alternative materials possess smooth surfaces and thus exhibit this disadvantage of low resistance to lateral movement. For example, plastic/plastic composite ties with smooth surfaces exhibit a single tie push test value of about 1000 pounds. This test is a railroad industry standard test for measuring a tie's resistance to lateral sliding when installed in ballast. A value of 1000 pounds is comparable to that of new wooden ties. However, as wooden ties are worked into the ballast, their single tie push test values increase. After the passage of about 15 million gross tons (MGT) of rail traffic, the single tie push test values for wooden ties increases to 2500–3000 pounds. Conversely, smooth plastic ties do not exhibit any appreciable increase in single tie push test values even after significant traffic has passed over them.
There is a cost which the railroads pay for the fact that new wooden ties have low mechanical interaction with the ballast (reflected by low single tie push test results) immediately upon installation. This price is that for safety reasons they necessarily require the trains to use lower speeds whenever new wooden ties are installed, especially around curves. This cost is particularly high in locations where the tracks go over passes in high mountain ranges. In some instances, at certain times of the year, trains cannot safely traverse these passes at all during the day. This disadvantage is associated not only with new wooden ties but also ties made from alternative materials such as plastic composites which have smooth surfaces.
Previous attempts have been made to texturize a surface of a railroad tie. These attempts involved scoring the surface with wavy line patterns, box patterns, or checkerboard patterns. However, these patterns do not provide an effective texture for interacting with the ballast of the railroad bed to inhibit sliding.